Implement new option 'mouse-prefer-closest-glyph'

Implement new option 'mouse-prefer-closest-glyph'

* src/dispnew.c (mouse_prefer_closest_glyph): New global
variable.
(buffer_posn_from_coords):
* src/xdisp.c (remember_mouse_glyph): Respect
'mouse_prefer_closest_glyph'.
(mouse_fine_grained_tracking): Update documentation to
include 'mouse_prefer_closest_glyph' effects.

* doc/lispref/commands.texi (Accessing Mouse): Update
documentation to say what the new option does when enabled.
* lisp/cus-start.el (standard): New user option
'mouse-prefer-closest-glyph'.